Linear stepper motor

ABSTRACT

A stepper motor includes a forcer wherein the teeth are formed in the pole faces by an etching process or machining and instepping. The etching can be achieved by forming the teeth in a separately formed plate which is anchored to the face of a forcer. A high density magnetic material is used for the plate and can also be used on the platen to improve performance of the stepper motor.

BACKGROUND OF THE INVENTION

This invention relates to linear stepper motors. In particular, theinvention is concerned with a system for forming components of thelinear stepper motor, and for enhancing the operational performance ofsuch motors.

Linear stepper motors are used for positioning applications requiringrapid acceleration and high speed moves with low mass payloads.Mechanical simplicity and precise open loop operation are additionalfeatures of the stepper linear motor systems.

A linear stepping motor operates on the same electromagnetic principlesas a rotary stepping motor. The stationary part or platen is a passivetoothed steel bar extending over the desired length of travel. Permanentmagnets, electromagnets with teeth, and bearings are incorporated intothe moving elements or forcer. The forcer moves bidirectionally alongthe platen, assuring discrete locations in response to the state of thecurrents in the field windings. In general, the motor is two-phase,however a larger number of phases can be employed.

A linear stepper motor is not subject to the same linear velocity andacceleration limitations inherent in systems converting rotary to linearmotion. With linear motors, the force generated by the motor isefficiently applied directly to the load and length has no effect onsystem inertia. Additional benefits of linear stepper motors include:

1. The motors are capable of speeds to 100 ips and the low mass forcerallows high acceleration.

2. The need for lead screws or belts and pulleys is eliminated.

3. Length of travel is limited by the length of the platen, andincreasing the platen length causes no degradation in performance.

4. Precise open loop operation is possible, and unidirectionalrepeatability to 2.5 micron is possible without the added expenses offeedback devices.

5. A linear motor is usually smaller in all three dimensions thancomparable systems where rotary motion is converted to linear.

6. More than one forcer can operate on the same platen with overlappingtrajectories.

A critical requirement to ensure the accuracy of stepper motors is tohave accurate positioning of the teeth in the platen and the teeth inthe forcer. Conventionally, it is possible to form the teeth in a platenby an etching process, and the nature of etching is such that theseteeth can be accurately positioned. This has not been possible, toaccurately form the teeth in the pole faces in the electromagnetsforming part of the forcer. Accordingly, a limitation exists in theaccuracy of linear stepper motors due to this characteristic.

There is accordingly a need to provide a stepper motor with enhancedability to accurately position the teeth in the forcer. Moreover, it isdesirable to provide systems to increase the magnetic flux density ofboth the platen and the teeth in the forcer.

SUMMARY OF THE INVENTION

According to the invention, there is provided a linear motor whichcomprises a forcer for movement over a platen. There are bearing meansbetween the platen and the forcer, and the platen has teeth directedtransversely to the direction of movement of the forcer over the platen.The forcer has a series of teeth directed towards the platen, and arefor substantially parallel relationship with the teeth and the platen.The teeth in the forcer are created by an etching process which providesfor increased accuracy and repeatability.

In one preferred form of the invention the teeth are formed by a platewhich is slotted and anchored to the forcer.

Another aspect of the invention is to have the material of the plate ofa substantially higher magnetic density than the pole faces forming theelectromagnets. The teeth for the platen can also be formed of amagnetic density substantially higher than the magnetic density of thematerial forming the base for the platen. The teeth of the platen couldbe formed of a plate to overlay the base.

The invention is concerned with stepper motors directed in the x axisand also in the x and y axes.

The invention is described further with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view, with portions shown in phantom of a forcerlocated on a platen of a stepper motor operable in an x direction.

FIGS. 2a-2d are diagrammatic illustrations of the forcer of a steppermotor in relationship to the teeth on a platen.

FIG. 3 is a perspective view of a platen and a housing for four forcersof a stepper motor for operating in the x and y directions.

FIG. 4a is a partially exploded underview, with part of a base platebroken away, of a housing showing four forcers constituting the steppermotor for operating in the x and y directions.

FIG. 4b is an underview of the base plate for the housing of the fourforcers constituting the stepper motor for operating in the x and ydirections. For each forcer there are strips of material between slotsand teeth.

FIG. 5 is an exploded view of a forcer showing the body, permanentmagnets, pole pieces and the slotted plate for forming the teeth on theforcer.

FIGS. 6a-6c are cross-sectional diagrammatic side views of forcersshowing different relationships of the slotted plate in relationship tothe body of the forcer.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, there is a platen 10 which is a steel bar having ateeth 11 on a planar surface. The teeth 11 are directed substantiallytransversely to the direction of movement of a forcer 12 which moves inthe x direction as indicated by arrow 13. The teeth 11 on the surfaceare all substantially parallel with each other and are parallel to teeth14 which are formed on the underside of pole faces 15 which constitutepart of the forcer 12.

The teeth 14 of the forcer 12 are directed towards the teeth 11 of theplaten 10 and are in a substantially parallel relationship to the teeth11 of the platen 10. The teeth 14 are formed in the face of the polefaces 15 in a manner that will be further described below. The forcer 12includes a body member 16 for the forcer 12 which locates field windings17 and permanent magnets 18.

The stepper motor illustrated in FIG. 1 also includes bearings 19, whichare mounted on the housing securing the forcer 12. These bearings 19 canbe either mechanical roller bearings or air bearings. Mechanicalbearings are simple, and the stiffness of the bearing reduces the pitchand roll of the forcer 12. Air bearings are free of mechanical friction,and operate by floating the forcer 12 on high pressure introducedthrough orifices near the pole faces 15 of the forcer 12. Theillustration shown represents mechanical bearings 19.

The operation of the linear stepper motor is now described withreference to the operation of the motor where there are twoelectromagnets in a two-phase motor. FIGS. 2a-2d do not illustrate theinvention. The linear stepper motor is microstepped by proportioningcurrent to the two phases of the forcer 12, in a manner similar torotary stepper motors. The forcer 12 consists of two or fourelectromagnets (A and B) and a strong rare earth permanent magnet 18. Ina two-phase motor there are two electromagnets; in a four-phase motorthere are four magnets; in a six-phase motor there are six magnets, etc.FIGS. 2a-2b illustrate two of the electromagnets 18 for simplicitypurposes. The two pole faces 15 of each electromagnet are toothed toconcentrate the magnetic flux. Four sets of teeth 14 on the forcer 12are spaced in quadrature so that only one set at a time can be alignedwith the platen teeth 11.

The magnetic flux passing between the forcer 12 and the platen 10 givesrise to a strong normal force of attraction between the two pieces. Theattractive force can be up to ten times the peak holding force of themotor. This requires a bearing arrangement to maintain precise clearancebetween the pole faces 15 and platen teeth 11.

When current is established in a field winding 17, the resultingmagnetic field tends to reinforce permanent magnetic flux at one poleface 15 and cancel it at the other. By reversing the current, thereinforcement and cancellation are exchanged. Removing current dividesthe permanent magnetic flux equally between the pole paces 15. Byselectively applying current to phase A and B, it is possible toconcentrate flux at any of the forcer's four pole faces 15. The face 15,receiving the highest flux concentration, attempts to align its teethwith the platen 10. FIGS. 2a-2d show the different flux paths, anddepict the four states or full steps of the forcer 12. The four stepsresult in motion of one tooth interval to the right. Reversing thesequence moves the forcer 12 to the left.

Repeating the sequence causes the forcer 12 to continue movement. Whenthe sequence is stopped, the forcer 12 stops with the appropriate toothset aligned. At rest, the forcer 12 develops a restoring or holdingforce which opposes any attempt to displace it from equilibrium. Therestoring force increases until the displacement reaches one-fourth of atooth interval.

In FIG. 3 there is illustrated a motor housing 20 which is operationalin both in x direction 21 and in y direction 22. The housing 20 has fourmotors 23, 24, 25, and 26 for moving over a platen 27 formed of magneticmaterials such as iron. There are two sets of parallel grooves 28 and29. Grooves 28 are directed in the x axis, and grooves 29 are directedin the y axis. The two sets of grooves 28 and 29 therefore providemagnetic teeth 30 having a square cross-section. The outstanding teeth30 and the grooves 28 and 29 provide for the magnetic energyconfiguration which is used in cooperation with the electromagneticmeans in the housing 30 as constituted by the four forcers 23, 24, 25,and 26. As such, they provide for movement of the head or housing 20 inthe x and y axes 21 and 22 respectively.

FIG. 4a illustrates the underside of the head or housing 20. There canbe seen a plurality of forcers 23, 24, 25, and 26 mounted in a base 70.Each forcer is like the forcer 12. The forcers 23, 24, 25, and 26 havemounted collectively in relation to a single plate 71 so that theycooperate with the platen 27. The electromagnetic means are divided hereto show the four pole faces A, B, C, and D, which represent the A, B, C,and D magnetic phases which are detailed more particularly in each ofthe four pairs of electromagnets shown in FIG. 1. With this arrangement,the housing or head 20 can move over the platen 27 according to theoperation of the respective forcers 23, 24, 25, and 26 in a similarmanner to that described with regard to FIGS. 2a-2d.

In FIG. 4b there is shown the single plate 71 which is for location onthe base 70. A suitable fixation means can be provided for securing theplate 71 with the base 70. By having all the teeth 14 formed by materialslots 51 cut into the single plate 71, rather than having the teeth setup individually for each of the four forcers, it is easier to obtain therelative registration of the four forcers relative to each other. Insome other cases there can be individual plates provided for each of theforcers. Each forcer 23, 24, 25, and 26 has an arrangement of teeth 14formed between slots 51 and between material 52 in a manner similar tothat shown in FIGS. 5, 6a, 6b, or 6c.

FIG. 5 illustrates one aspect of the inventive concept in forming theforcer 12. For either a linear stepper motor to operate in the xdirection 21 or multiple forcers to have a linear stepper motor operatein the x and y directions 21 and 22 respectively, the forcer 12 has abody member 16 with four compartments 40, 41, 42, and 43 respectively.Inside each of these compartments there is located an electromagneticconfiguration which has four pole pieces 44 directed outwardly from thecompartment 40 such that there are pole faces 45 directed to thesubstantially flush with the top phase 46 of the body 16. Within thespace 47 there is a suitable field winding 17 that is located betweenthe two sets of pole pieces 44. In each of the compartments, there is apermanent magnet 18. Four of these constructions are formed in the bodymember 16 to create the forcer 12.

Conventional practice has been to form in the poles faces 45 teeth whichare substantially parallel to the teeth in a platen. By the invention,however, there is provided a separate plate 50 where there are providedspaced slots arranged in sets of four corresponding to the faces 45 ofthe each of the pole pieces 44. The pole pieces 44 themselves are flatand not provided with the teeth. The teeth 14 are effectivelyconstituted by the strips of material 52 between each of the slots 51.Around the slots 51 there is a perimeter of material 53, and thismaterial is anchored to the face 46 of the top of the body 16.

By this arrangement, the plate 50 is suitably etched through aphotoetching process so that the slots 51 can be extremely accuratelyformed in the face 46 of the plate 50. In this manner, there is provideda system for forming accurately spaced teeth to constitute the polefaces 44 of the forcer 12. Instead of etching, this can be done bymachining, namely cutting or stamping.

To complete the construction of the face for adjacency with the teeth ofthe platen, the slots 51 are epoxy filled. The exposed face of the plate50 is subjected to grinding to provide a smooth surface. Thereafter,diamond lapping can be provided to provide a high quality finish to theproduct.

The material forming the plate 50 can be high flux density VanadiumPermidor™ which has a maximum flux density of about 21,000 gauss. Thiswould compare with the conventional steel which has a flux density ofabout 14,000 gauss, and which is the normal material from which a polepiece is made. The teeth flux density is twice that of the poles andback-iron. Since Vanadium Permidor™ is considerably more expensive thansteel, it is now possible to provide teeth of higher flux densitythereby providing an effective forcer having an effective high fluxdensity concentration where it is needed, namely, in the zones of theteeth, but not in the pole pieces themselves. Thus, although VanadiumPermidor™ is a considerably more expensive material, the net result is acombination product which is high quality, high performance, andconsiderably less a cost than if the entire product was made of a highermagnetic density material. The motor force is proportional to the squareof the tooth flux density.

For the motor to operate in the high force mode, both platen and forcerteeth are made of hi-flux density material such as Vanadium Permidor™.Since in a 0.040 in. pitch motor the etched plates on both the forcerand the platen are less than 0.020 in. thick, the cost of using VanadiumPermidor™ is relatively small.

Similarly, the teeth 30 of the platen 27, or the teeth 11 of the platen10, can be made of a high magnetic flux density material, whereas therelative bases can be of a lower flux density material. Overall, thereis therefore provided a system for interacting a forcer with a platenwith components having a high flux density characteristic in a mannerwhich has not previously before been anticipated. The teeth for theplaten can be made in a plate which is then bonded to the base member.

FIGS. 6a, 6b, and 6c illustrate three of the different embodiments bywhich the plate 50 can be formed relative to the face 46 on the forcer12.

In FIG. 6a there is shown strips 52 with the epoxy etching material inwhat would otherwise have been the slots 51. The embodiment shown inFIG. 6b shows a partially etched plate with epoxy 60 filling the spacesbetween the teeth 52. There is still retained a thin sliver of material61 below each of the slots 51 which is filled with epoxy 60. Theadvantage of this arrangement is to prevent the buckling of the thinstrips 52 in the plate, especially when the plate is put into adjacencywith the high magnetic field elements in the forcer body 12. In FIG. 6cthe arrangement is shown where a sliver of material is retained. Thesliver of material 62 is on the top of the teeth. All strips of material52 face downwardly. The sliver 62 is thereafter ground off, leaving thestrips of material 52 and epoxy 60.

Many other forms of the invention exist each differing from others. Theinvention is to be determined solely by the following claims.

What is claimed is:
 1. A linear motor comprising:a forcer for movementover a platen, bearing means between the platen and the forcer, theplaten having teeth directed transversely to the direction of movementof the forcer over the platen, the forcer having a series of teethdirected towards the platen and being for substantially parallelrelationship with the teeth in the platen, and the teeth in the forcerbeing formed by a plate which is slotted and anchored to a face of abody member of the forcer for locating field windings, electromagnets,and a permanent magnet, and wherein the electromagnets form pole facesand the slots of the plate align with the pole faces.
 2. A motor asclaimed in claim 1 wherein the plate is formed of a material of magneticdensity substantially higher than the magnetic density of the polefaces.
 3. A motor as claimed in claim 2 wherein platen includes a baseand the teeth in the platen are formed of a material having a magneticdensity substantially higher than the magnetic density of the base.
 4. Amotor as claimed in claim 1 wherein the platen includes a base and theteeth in the platen are formed of a material having a magnetic densitysubstantially higher than the magnetic density of the base.
 5. A motoras claimed in claim 1 including a filler material in the slots betweenthe teeth.
 6. A motor as claimed in claim 2 including a filler materialin the slots between the teeth.
 7. A motor as claimed in claim 1 whereinthe slots are formed by at least partial etching.
 8. A motor as claimedin claim 1 wherein the slots are etched in the plate so as to form slotsthrough the plate.
 9. A linear motor as claimed in claim 1 wherein theteeth in the forcer are created selectively by a process of etching ormachining.
 10. A motor as claimed in claim 1 mounted in a housing, andincluding at least multiple other forcers mounted in the housing wherebymotion of the housing is effected in a x direction and y direction, andthe platen includes teeth transversely directed in the x and ydirections.
 11. A forcer for a linear motor having the forcer formovement over a platen, bearing means between the platen and the forcer,the platen having teeth directed transversely to the direction ofmovement of the forcer over the platen, the forcer having a series ofteeth directed towards the platen and being for substantially parallelrelationship with the teeth in the platen, and wherein the forcerincludes a plate which is slotted to form the teeth, the plate beinganchored to a face of a body member for locating field windings,electromagnets, and a permanent magnet, and wherein the electromagnetsform pole pieces and the slots of the plate align with faces of the polepieces.
 12. A forcer as claimed in claim 11 wherein the plate is formedof a material of magnetic density substantially higher than the magneticdensity of the pole pieces.
 13. A forcer as claimed in claim 11 whereinthe plate is formed of a material of magnetic density substantiallyhigher than the magnetic density of the pole pieces.
 14. A forcer asclaimed in claim 11 including a filler material in the slots between theteeth.
 15. A forcer as claimed in claim 11 wherein the teeth arepartially etched.
 16. A forcer as claimed in claim 11 wherein the teethare at least fully etched in the plate so as to form slots through theplate.
 17. A forcer as claimed in claim 11 mounted in a housing, andincluding at least one other forcer is mounted in the housing wherebymotion of the housing is effected in an x direction.
 18. A forcer asclaimed in claim 11 mounted in a housing, and including at leastmultiple other forcers mounted in the housing whereby motion of thehousing is effected in a x direction and a y direction, and the platenincludes teeth transversely directed in the x and y directions.
 19. Amethod of manufacturing a forcer for a linear motor comprising providinga plate with slots to form teeth between the slots, anchoring the plateto a face of a body member for locating field windings, electromagnets,and a permanent magnet, the electromagnets forming pole pieces, andaligning the slots of the plate with faces of the pole pieces.
 20. Amethod as claimed in claim 19 wherein the plate is formed of a materialof magnetic density substantially higher than the magnetic density ofthe pole pieces.
 21. A method as claimed in claim 19 wherein the plateis formed of a material of magnetic density substantially higher thanthe magnetic density of the pole pieces.
 22. A method as claimed inclaim 19 including a filler material in the slots between the teeth. 23.A method as claimed in claim 19 wherein the teeth are partially etchedin the plate.
 24. A method as claimed in claim 19 wherein the teeth areat least fully etched in the plate so as to form slots through theplate.
 25. A method as claimed in claim 19 including the step ofmanufacturing a platen for the linear motor comprising forming theplaten to include a base and teeth, and wherein the teeth in the platenare formed of a material having a magnetic density substantially higherthan the magnetic density of material of the base.
 26. A method asclaimed in claim 25 wherein the platen includes teeth transverselydirected in the x direction.
 27. A method as claimed in claim 25 whereinthe platen includes teeth transversely directed in the x and ydirections.